That episode of Big Bang never addressed the obvious discrepancy.
As physicists they most likely would have believed in the concept of spacetime. Spacetime is the concept that space began with a Big Bang and has been expanding ever since, forever tying space and time together. As the universe expands every second it adds not just size but also time.
The leading edge of the expansion the universe is what we, as humans, refer to as "now". If true, then there is no future place to travel to and traveling back in time is also impossible since you would have to also destroy space (shrink the universe) to achieve it.